The present invention relates to a hydraulic anti-skid apparatus for installation in a vehicle braking system between a master cylinder and a wheel brake cylinder to prevent the road wheels of the vehicle from locking in braking operation.
As one of such anti-skid apparatuses as described above, there has been proposed an anti-skid apparatus which comprises a cut-off valve disposed within a braking circuit connecting a master cylinder to a wheel brake cylinder, a bypass valve disposed within a bypass passage of the braking circuit, first and second pistons respectively arranged to control opening and closing operations of the cut-off valve and the bypass valve, the first and second piston each being arranged to be applied at one end thereof with a hydraulic braking pressure from the master cylinder and at the other end thereof with a hydraulic power pressure from a fluid pump, a solenoid valve arranged to be activated when a road wheel of the vehicle tends to be locked in braking operation, the solenoid valve being adapted to apply the hydraulic power pressure to the first piston in its deenergized condition and to connect the first piston to a fluid reservoir in its energized condition, and a regulator valve arranged to control the hydraulic power pressure applied to the first and second pistons in accordance with the hydraulic braking pressure applied thereto from the master cylinder.
In the above-described conventional anti-skid apparatus, the first and second pistons may not be maintained in their initial positions during deactivation of the fluid pump. If the master cylinder is actuated during deactivation of the fluid pump, the first and second pistons are displaced from their initial positions by the hydraulic braking pressure to cause abnormal operations of the cut-off valve and the bypass valve. For this reason, the fluid pump must be always driven during travel of the vehicle. This results in lose of the power and a decrease in the life-span of the fluid pump. Furthermore, in the conventional anti-skid apparatus, the first piston is moved backward by the hydraulic braking pressure when connected to the fluid reservoir in response to energization of the solenoid valve, and subsequently the first piston is rapidly supplied with pressurized fluid from the regulator valve in response to deenergization of the solenoid valve. This causes pressure pulsation in the regulator valve and the master cylinder, resulting in unpleasant shocks on the brake pedal.